DESCRIPTION (adapted from the applicant's description): Removal of introns from precursor mRNA by splicing is mandatory for expression of most genes in higher eukaryotes. Recent work defined a rare but broadly distributed class of introns that are spliced by an alternative pathway not used for typical introns. Both the major and minor classes of introns are spliced by a similar mechanism in a large ribonucleoprotein complex termed the spliceosome, but the spliceosomes for each class differ in RNA composition while maintaining a high degree of similarity in critical RNA-RNA interactions. By contrast to the major class spliceosome, very little is known about the protein composition of the minor class spliceosome. This project seeks to investigate the roles of known splicing factors of the major intron class in splicing of minor class introns, using primarily biochemical approaches in a reconstituted splicing system. Factos involved in early steps of splice site selection are of particular interest because the two intron classes likely communicate with each other to allow correct splicing of messages containing both kinds of introns. Shared and class-specific spliceosome proteins will be identified by analyzing affinity selected minor class spliceosomes with appropriate antibodies, and by obtaining sequence information for proteins recovered from the purified minor class spliceosomes.